Exposure of F9 teratocarcinoma cells to retinoic acid induces differentiation to a parietal endoderm cell type. Cyclic AMP-dependent protein kinase activity of the plasma membrane fraction is enhanced within 3 h of retinoid treatment of F9 cells and reaches a maximum at 15 h after the addition of retinoic acid. Retinoic acid treatment elicits a preferential increase in the amount of the R II regulatory subunit of the protein kinase associated with the membrane fraction. This suggests that the specific increase in type II cyclic AMP-dependent protein kinase associated with the plasma membrane may be an early event of retinoic acid action. The amount of a transformation sensitive 55 K protein decreases in F9 cells induced to differentiate with retinoic acid. Results suggest that a decrease in the amount of 55 K protein correlates with the onset of differentiation and not with cell growth rate. Low concentrations of insulin (20 to 40 ng/ml) stimulate the growth of F9 cells under defined, serum-free conditions. Insulin appears to stimulate the growth of these cells by acting directly through its own receptor.